This invention relates to a vital switch control circuit for use at a railroad installation where immunity to stray or induced a.c. currents and low-level d.c. signals is required. More specifically, this invention relates to a vital switch control circuit which advantageously utilizes a permanent magnet motor to provide immunity to the stray or a.c. stray or induced a.c. signals and a mechanically-interlocked, dual-coil, reversing contactor device, hereinafter referred to as a reversing contactor for providing immunity to the low-level d.c. signals.
The railroad switch machine, for which the subject vital switch control circuit is designed, operates to move the switch points between their two extreme positions. Such switch machines accomplish this operation typically either manually, electrically, or, often, a combination of both. When the switch machine can be electrically operated, such energy is predominantly a d.c. type power. Whichever type switch machine is used, however, precautions must be taken to avoid inadvertent switch operation caused by stray or induced a.c. signals which may be present at the railroad installation from a number of sources, including commercial a.c. power lines and a.c. transmission conductors used for vehicle traction power. Additionally, typical railroad practice is to run control cables from a central control location; for instance, a wayside control case, through a single buried conduit, to the various signal and switch devices to be controlled therefrom. Such practice, though economical in terms of initial installation costs, suffers the inherent disadvantage that, in the event of a grounding or short-circuit condition arising within the single conduit, low-level d.c. signals used for traffic control may cross over to the switch control lines. Additional d.c. interference may arise where the vehicle traction power is d.c. To prevent such an undesired signal cross-over from inadvertently throwing a switch machine under a moving train, for instance, elaborate check schemes between the switch control vital components and the motor which effects switch movements are required. These check schemes further complicate the installation and maintenance operations by requiring additional connecting lines between the control locations and the switch machine. Since the distance between the control location and the switch machine can, at times, be quite large, an additional cost arises for such installation and maintenance operations.